I believe that 1:9 should be good up to about 69-70 gns maybe even 75 Gn, depending on the actual barrel twist, length of bullet and velocity. Different barrel manufacturers have slightly different tolerances on the twist rate. Some might get 1:9.3 say while others at 1:8.7, just for instance. I have had decent luck stabilizing 68 GN HPBT with a 1:9 twist in an AR.
Also, the longer the bullet, the faster the twist is needed. Generally a heavy bullet in the same caliber will be longer than a lighter bullet but not always. Using 303 as an example a 174 Gn HPBT is a lot longer than a 174 Gn RN and thus requires a faster twist to stabilize. However I believe that bullets are more tolerant of being over spun than under spun, generally. If I were you, I would not worry about getting too fast a spin in a factory barrel. Most us 1:9 as a standard now. There are a few with 1:7 and even fewer with 1:12 I believe.
Also, as I understand it, the faster a bullet is driven down the barrel, the faster it will spin in a given twist. Thus if you drive it hard, it will stabilize better. I think. I am reaching a bit on that one.
Ultimately, in order to see what your rifle / barrel likes, you have to try different ammo/bullets/powders to see what works. If you are looking for premium performance in factory ammo, expect to pay more $$$. Budget packs of Winchester white box will not likely have have heavy, long range bullets. Frankly it's not something I keep up on. I always roll my own
The most important thing is to get out and shoot